As the one-year anniversary of Hurricane Katrina is upon us, it becomes increasingly important to understand not only the scientific origins of hurricanes, but also ways to learn from the past to better predict and plan for hurricanes in the future.
Hurricane Katrina was the one of the most devastating natural disasters in United States history, causing 1833 fatalities and over $81 billion in damage. Even though it was only a Category 3 hurricane by the time it made landfall near New Orleans on August 29, 2005, the damage was so extensive that is still not close to being understood even a year later.
Katrina's storm surge flooded a number of coastal towns and caused the breaching of levees separating Lake Pontchartrain from New Orleans, resulting in the flooding of 80% of the city.
The storm surge caused severe or catastrophic damage along the Gulf coast, devastating the cities of Mobile, Alabama, Waveland and Biloxi/Gulfport in Mississippi, and Slidell and other towns in Louisiana. Severe wind damage was reported well inland.
What is a Hurricane?
A hurricane on Earth is a severe storm that forms from warm tropical oceans, moisture and light winds above them. If the right conditions last long enough, a hurricane can produce violent winds, incredible waves, torrential rains and floods. In other regions of the world, these types of storms have different names.
- "Hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the International Dateline, or the South Pacific Ocean east of 160?E)
Typhoon" (the Northwest Pacific Ocean west of the International Dateline)
- "Severe Tropical Cyclone" (the Southwest Pacific Ocean west of 160?E or Southeast Indian Ocean east of 90?E)
- "Severe Cyclonic Storm" (the North Indian Ocean)
- "Tropical Cyclone" (the Southwest Indian Ocean)
What Causes a Hurricane?
Warm ocean waters drive hurricanes. A hurricane requires surface sea temperatures of 82 degrees or warmer to gain energy and gather its damaging strength. Warmer water temperatures -- several studies have shown that ocean temperatures worldwide are climbing with atmospheric concentrations of greenhouse gases -- could well expand the current range of coastline affected by hurricanes.
Hurricanes rotate in a counterclockwise direction around an "eye." A tropical storm becomes a hurricane when winds reach 74 mph. In the United States, there are an average of six Atlantic hurricanes each year; over a three-year period, approximately five hurricanes strike the United States coastline from Texas to Maine. The Atlantic hurricane season begins June 1 and ends November 30. The East Pacific hurricane season runs from May 15 through November 30, with peak activity occurring during July through September. In a normal season, the East Pacific would expect 15 or 16 tropical storms. Nine of these would become hurricanes, of which four or five would be major hurricanes.1
When hurricanes move onto land, the heavy rain, strong winds and heavy waves can damage buildings, trees and cars. The heavy waves are called a storm surge.
A storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level by 15 feet or more. In addition, wind waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm tides is tremendous.
The level of the storm surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast (right, top picture) will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf (right, bottom picture) will not see as much surge inundation, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors severely damage ships, marinas, and pleasure boats.2
The highest storm surge ever recorded was produced by the 1899 Cyclone Mahina, which caused a 13-meter (43 feet) storm surge at Bathurst Bay, Australia. In the United States, the greatest recorded storm surge was generated by Hurricane Katrina, which produced a storm surge 9 meters (30 feet) high in the town of Bay St. Louis, Mississippi, and in the surrounding coastal counties. The worst storm surge, in terms of loss of life, was the 1970 Bhola cyclone and in general the Bay of Bengal is particularly prone to tidal surges.
Predicting and Tracking Hurricanes
Remembering the devastation caused by Hurricane Katrina and the enormity of the damage that had been caused by other hurricanes has created a new awareness of hurricanes and has prompted scientists around the globe to gain a better understanding of these deadly storms in an effort to predict and mitigate future similar catastrophic events.
Toward this goal, the National Hurricane Center (NHC), part of the National Oceanic and Atmospheric Administration (NOAA), NASA and other federal agencies work together to provide the public with the best predictions and information possible. The NHC uses several computer models to help forecast and track the intensity of tropical cyclones. Each computer model includes air temperature and pressure, sea surface temperature, wind speed and humidity as recorded from hurricane hunter aircraft that fly above tropical cyclones and drop sensors into them to obtain data. The NHC also verifies storm locations with NOAA's Geostationary Operational Environmental Satellites (GOES).
NASA's Contributions Include 3 Earth-watching Satellites.
This image from Aqua's AMSR-E instrument shows sea surface temperature conditions centered in the Atlantic and the Gulf of Mexico on July 17, 2006, compared to average conditions (1985-1997) determined from data from the Advanced Very High Resolution Radiometer (AVHRR) on NOAA satellites. Places where temperatures were cooler than average are blue, places where they are warmer than average are red, and places where temperatures were around average are white. A swath of cooler-than-average temperatures arcs northeast away from southern North America, while to the south, a wide region of warmer-than-normal temperatures stretches between Africa and Central America. These warmer-than-normal sea surface temperatures in what is known as "Hurricane Alley" will become important as the hurricane season progresses.
NASA's Tropical Rainfall Measuring Mission (TRMM), QuikSCAT, and Aqua satellites each look at different factors of tropical cyclones to help generate better forecasts. TRMM focuses on the intensity of tropical rainfall, which is indicative of whether a cyclone is weakening or strengthening. QuikSCAT collects wind data, and Aqua records ocean and air temperatures and humidity. These factors are primary in the strengthening of a hurricane, and NASA researchers, working with hurricane forecasters from the NHC hope that the data generated from these satellites will improve hurricane predictions.
Hopefully, these efforts will help lessen damages when another hurricane, such as Andrew in 1992, or Katrina strikes United States coastlines.
1 NOAA: http://hurricanes.noaa.gov/
2 NOAA: http://hurricanes.noaa.gov/prepare/surge.htm
Neumann, C.J. (1993): "Global Overview" - Chapter 1" Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland
NASA's Goddard Space Flight Center: (from 1992) http://www.gsfc.nasa.gov/topstory/20020823andrew.html
Last Updated: 22 February 2011